The dispute over the 24-week abortion limit has been fuelled by the publication of figures from a leading premature-baby unit showing a dramatic increase in survival of babies born at 25 weeks or under.

From 1981 to 2000, the proportion of babies who survived after being born at 22-25 weeks gestation more than doubled from 32 per cent to 71 per cent at the University College London Hospitals NHS Trust.

Pressure has been growing for a cut in the 24-week limit because of evidence that some babies are viable at gestations of 23 or 22 weeks. MPs have been promised a free vote on any amendments on abortion to the Human Fertilisation and Embryo Bill currently before Parliament.

Professor John Wyatt, a consultant neo-natologist at UCLH, who led the study, published in Acta Paediatrica, said the survival rate for the unit was substantially higher than reported nationally. "We fully acknowledge that single-centre studies such as this one have statistical limitations because of the relatively small numbers involved. However, they are hugely important because they provide information on the survival rates that can be achieved with consistent levels of staffing and resources and with consistent policies."

Professor Wyatt was criticised by the Commons Science and Technology Committee last year for giving evidence that 42 per cent of babies born in his unit at 23 weeks showed signs of life. He later admitted that the figure had not been published in a peer- reviewed journal.